The development of fully integrated radio-frequency front-ends used in cell phones, radios, and other devices, requires the adoption of high performance filtering systems that could be integrated on the same chip as antenna switches and power amplifiers.
Micro-Electro Mechanical (MEM) resonators and filters represent promising candidates for replacing Surface Acoustic Wave (SAW) devices in future transmitter and receiver modules. Unlike SAW-based devices, MEM resonators and filters can be built using CMOS-compatible fabrication processes, thereby enabling integrated RF front-ends. However, to make migration to MEM-based devices feasible, it is necessary to improve the performance attained by MEM resonators and filters so as they are on par with SAW-based devices.
Aluminum nitride (AlN) based piezoelectric MEM resonators have been largely researched for their ability to achieve moderate quality factor (Qs) and high electromechanical coupling coefficient (kt2) throughout the entire microwave spectrum. In particular two main types of AlN based resonators have been already demonstrated: Film Bulk Acoustic Resonators (FBARs) and contour mode resonators (CMRs). FBARs use the AlN d33 piezoelectric coefficient to excite longitudinal vibrations along the thickness of AlN plates. In contrast, CMRs can excite lateral vibrations along one of the in-plane dimensions of AlN plates through the AlN d31 piezoelectric coefficient. As the electromechanical coupling of a MEM resonator is proportional to the magnitude of the adopted piezoelectric coefficient, FBARs show larger kt2 than CMRs. In contrast, CMRs enable multiple frequency references on the same chip, without additional fabrication costs.
Although the excitation of combined modes was proposed as a way to increase kt2 in AlN piezoelectric resonators, previous work showed only small improvements with respect to that attained by CMRs. In addition, devices demonstrated in previous work cannot attain a large capacitance per unit area, thereby complicating its impedance matching to a 50-ohm load.